Semiconductor devices that use nitride semiconductors such as GaN, GaInN, AlGaN and AlGaInN are being put to practical use. Typical devices are light-emitting devices such as light-emitting diodes and laser diodes in which a double hetero p-n junction-type light-emitting structure is constructed of a nitride semiconductor. In particular, nitride-type LEDs having a light-emitting structure that has been heteroepitaxially grown on a c-plane sapphire substrate are mass-produced as a light source for backlighting and illumination.
The nitride semiconductor crystals which are used in light-emitting devices have a hexagonal wurtzite structure. It has been reported that because the above-described nitride LEDs which employ a c-plane sapphire substrate have a light-emitting structure wherein the n-type layer, the active layer and the p-type layer making up the light-emitting structure are stacked in the c-axis direction, a piezoelectric field that lowers the light-emitting efficiency forms at the interior of the active layer. One approach that has been proposed for resolving this problem is a non-polar or semi-polar nitride LED obtained by homoepitaxially growing a light-emitting structure on a non-polar or semi-polar GaN substrate (Non-Patent Documents 1, 2, 3 and 4).
As used herein, “non-polar GaN substrate” refers to a GaN substrate whose principal plane is a plane that is tilted 90° to the c plane (such as an m plane or a plane, which plane is called a “non-polar face”). Also, “half-polar GaN substrate” refers to a GaN substrate whose principal plane is a plane having a slope with respect to the c plane which is smaller or larger than a non-polar face (which plane is called a “semi-polar face”). However, a GaN substrate in which the principal plane has a slight slope to the c plane is generally treated as a misoriented c-plane substrate. Nitride semiconductor substrates typically have a misorientation of within 10 degrees.
In nitride LEDs having a light-emitting structure on the Ga-polar face (c+ plane) of a c-plane GaN substrate, attempts are being carried out to improve the light extraction efficiency by using wet etching treatment such as photoelectrochemical (PEC) etching to roughen the substrate back face (N-polar face; c− plane) utilized as the light extraction plane. On the other hand, in nitride LEDs which use a non-polar or half-polar GaN substrate, it has been pointed out that the back face of the substrate cannot be roughened by wet etching (Non-Patent Document 5). Hence, methods that employ dry etching to roughen the back face of the substrate are being investigated for such nitride LEDs (Patent Document 1, Non-Patent Documents 5 and 6).    Patent Document 1: WO 2009/070809    Non-Patent Document 1: A. Chakraborty, B. A. Haskell, S. Keller, J. S. Speck, S. P. DenBaars, S. Nakamura and U. K. Mishra: Japanese Journal of Applied Physics 44 (2005), L173    Non-Patent Document 2: K. Okamoto, H. Ohta, D. Nakagawa, M. Sonobe, J. Ichihara and H. Takasu: Japanese Journal of Applied Physics 45 (2006), L1197    Non-Patent Document 3: R. B. Chung, Y-D. Lin, I. Koslow, N. Pfaff, H. Ohta, J. Ha, S. P. DenBaars and S. Nakamura: Japanese Journal of Applied Physics 49 (2010), 07020    Non-Patent Document 4: I. L. Koslow, J. Sonoda, R. B. Chung, C-C. Pan, S. Brinkley, H. Ohta, S. Nakamura and S. P. DenBaars: Japanese Journal of Applied Physics 49 (2010), 080203    Non-Patent Document 5: H. Zhong, A. Tyagi, N. Pfaff, M. Saito, K. Fujito, J. S. Speck, S. P. DenBaars and S. Nakamura: Japanese Journal of Applied Physics 48 (2009), 030201    Non-Patent Document 6: Y. Zhao, J. Sonoda, C-C. Pan, S. Brinkley, I. Koslow, K. Fujito, H. Ohta, S. P. DenBaars and S. Nakamura: Applied Physics Express 3 (2010) 102101